My charging system voltage is to low. after being started for a short time and also when the key in the on position my battery gauge will drop and my check gauges lite comes on. Where do I start

Start with the basics, is the battery in good shape and fully charged ? If so check the starter.

driving , or parked, engine on or off. (sure key on)
please post full clear symptoms.
post all the car does good and bad.
parked, idling and driving.

my guess is, you were driving, then looked at the cluster

1. and the fuel level dropped fast. (power loss 12vdc that is)
2. and the voltage gauge, dropped,(well it didnt lie , it did drop)
3. this means the battery charge is too low, and falling.
4. the battery is bad. or
5. the charge system( I never said to replace a part) is bad.
6. or the battery lugs are rusty. clean them.

battery is 12.6vdc rested, engine off
battery volts running is 13.3 to 15vdc, , if not charge system is upset or battery so bad the charge system shuts down.

If you swap the battery positions, does the problem move to the RHS ?

You've got a good start measuring the voltage at the battery. Looks like you've got a low voltage problem. Common on the GM side post batteries is battery acid leaking at the positive battery cable post on the battery. I would recommend removing and checking the cables/posts on the battery. If everything is clean, reassemble the battery cables to the battery. If there is excessive wetness or corrosion on EITHER of the cables/posts, you should replace the battery first, and clean the cables. A leaking battery is going to give you more trouble down the road if you just clean it. If the battery connections on the existing battery are clean, charge the battery with a battery charger, or jump start the car. Use your trusty meter and measure voltage at the battery with the engine running. (Also do this if you replace the battery for leaks). You should read in the neighborhood of 13.5 - 14.0 volts if your charging system is working correctly. If you are reading in the low 12 volt range with the engine running, something is wrong with your charging system, and most likely putting a strain on the battery, preventing it from receiving the charge it needs. If your existing battery is not leaking, cables are clean and tight, jump starts okay, reads 13.5-14ish volts with the engine running, but refuses to crank the engine over after the engine runs for about 15-20 minutes, it's time for a new battery.

The first step is to see if the gauge are correct,low voltage can cause many problems, the first step is to make sure the coolant is full so check radiator, this is done when cold, if full then start the engine with a meter on the battery and watch the voltage, have someone put it into drive while holding brake. if voltage on meter starts to drop then give a little gas to rev engine and see if voltage goes up, if it does then you may have to low an idle but if the idle is ok then alternator is bad. also have the battery tested to make sure its good, just because it starts the truck does not mean its good. the next thing is to look at the overheating problem. if the cooling system is full and it runs hot then that must be looked at,when the truck is starting to overheat we assume it is due to gauge, but this sounds like it may be low coolant or low voltage or idle is to low because when you put it in park it drops. start with these checks and let me know the results. good day.

THAT SOUNDS LIKE LOW VOLTAGE. PUT A VOLTMETER ON THE BATTERY WHILE TURNING THE KEY IN THE START POSITION. IF THE VOLTAGE DROPS BELOW 8 VOLTS, THAT IS TOO LOW. BATTERY NEEDS TO BE CHARGED OR REPLACED & TERMINALS CLEANED. IF IT STARTS CHECK THE CHARGING SYSTEM. MOTOR RUNNING, VOLTAGE AT THE BATTERY SHOULD READ 13.90 TO 14.90. IF IT IS TOO LOW GET THE ALTERNATOR TESTED.

is the alternator charging??if you have a battery charge light ..put ignition in start position..battery light should come on..if it dosnt the volatage regulator is shorted..if it does come on start engine..it should go off once engine has started.. buy a cheap dc voltage meter..radio shack 6.00 ..start engine,, get your voltage meter put red lead on battery positive black on neg.while engine is idleing it should read 13 to 14 volts..if you have a charge gauge on your dash check to see what your reading is..put headlights on at idle if it goes below 12 volts..its not charging..unless you have a short..then battery will lose charge overnight or after a few hours depending on short..that should cover most of it..happing hunting

Two obvious possibilities; The starter is binding, and takes all the battery has to turn it over, or...
There's a low-amp short draining the charge from the battery while the truck is parked.

Most electrical shops will do a complete system diagnostic for less than $20, be sure to let them know that you're looking for an ambient short.

The starter could just be gunked-up and need a good cleaning, but otherwise it's fine and can be saved...it's location makes it a good target for stray oil and dirt accumulation.

If you test battery with a load test it will probably fail. Battery is bad. Also have alternator tested. Go to any parts store, even walmart they all test for free. I would put my money on bad battery. Cells getting weak.

2 things have the battery load tested twice with a real carbon pile load tester under a 15 second load if battery still shows good what i find is oil has gotten into starter because oil filter sits over starter and people let it drip on the starter